The ionising radiation and gas-phase metallicity in the narrow-line regions of Seyfert galaxies

Abstract

The physical conditions in the narrow-line regions (NLRs) of active galactic nuclei (AGN) may be constrained by comparing spectroscopic observations with photoionisation models. The key parameters are the gas-phase abundances, the ionisation parameter, the gas pressure, and the shape of the ionising spectrum. The first part of this thesis presents a model of the ionising continuum radiation in Seyfert AGN. This model, OXAF, preserves the physically-motivated shapes of existing theoretical models while simplifying them to a three-parameter form designed for photoionisation modelling. The second part of the thesis presents the full data release of the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7). Three-dimensional data cubes, two-dimensional emission-line fits, and other products are provided to the astronomical community, along with a large number of maps of gas kinematics across the S7 sample. In the third part of the thesis, a Bayesian parameter estimation code is presented and used to compare photoionisation model grids with observed emission line fluxes. This code, NebulaBayes, is agnostic to model parameters, dimensionality, and the chosen emission lines. Grids of MAPPINGS photoionisation models are calculated and used with NebulaBayes to study extended NLRs in two dimensions across four 'pure Seyfert' S7 galaxies. The results provide the first robust two-dimensional measurements of gas-phase metallicity in Seyfert NLRs, and show steep metallicity gradients into extraplanar ionisation cones in the case of edge-on galaxy NGC2992, and an inverse metallicity gradient in the galaxy ESO138-G01. The near-constant ionisation parameter measurements suggest that radiation pressure regulates the density structure of NLRs on multi-kiloparsec scales. The ionising radiation is measured to be harder in ionisation cones than elsewhere, but the results are sensitive to spectral contamination. The fourth part of this thesis addresses mixing of NLR emission and HII-region emission in star-forming galaxies. The NebulaBayes code is coupled with a 'mixing' grid (combining HII and NLR model grids) and applied to a statistically powerful sample from the Sloan Digital Sky Survey (SDSS), thereby measuring the HII-AGN 'mixing fractions' in SDSS for the first time. For the majority of Seyfert-classified SDSS spectra, the majority of the Balmer emission is found to arise in HII regions as opposed to NLRs. The final results presented in the thesis are the first systematic and robust measurements of the gas-phase metallicities of active galaxies in the SDSS. The SDSS AGN follow a locus in mass-metallicity space that parallels the high-mass end of the star-forming galaxy sequence. We measure a ~0.1 dex increase in oxygen abundance over the observed range of AGN host galaxy masses.